In recent years, materials have become known—especially for organic light-emitting diodes—which significantly improve hole injection as well as hole transport in OLED's. (Lit.: Gufeng He, Martin Pfeiffer, Karl Leo, Appl. Phys. Lett. 85 (2004) 3911-3913).
This also lowers the operating voltage without affecting the efficiency of the OLED. These materials are strong electron acceptors doped in low quantities in the hole transporting layer of the OLED. Such additions facilitate the oxidation of the hole transporting material (i.e. the formation of holes), which otherwise is caused by the energy of the electrical field only. Hence, a weaker electric field (corresponding to a lower operating voltage) provides the same efficiency.
The drawbacks of this method for improved hole transport (also called p-doping) are the physical properties of the usable materials during the deposition process. These materials are fluorinated tetra-cyano-chinodimethanes, whose volatility is very difficult to control, so that these types of doping materials cannot be used in mass production since the material would contaminate the system due to uncontrollable distribution.